docs: def _fetch_pdb(pdb_code):
          """Load PDB file from rcsb.org."""
          txt = None
          url = 'http://www.rcsb.org/pdb/files/%s.pdb' % pdb_code.lower()
          try:
              response = urlopen(url)
              txt = response.read()
              if sys.version_info[0] >= 3:
                  txt = txt.decode('utf-8')
              else:
                  txt = txt.encode('ascii')
          except HTTPError as e:
              print('HTTP Error %s' % e.code)
          except URLError as e:
              print('URL Error %s' % e.args)
          return url, txt